1. Field of the Invention
The present invention relates in general to methods for passivating semiconductor device structures during fabrication thereof and, more particularly, to methods for passivating semiconductor-insulator interfaces in semiconductor device structures.
2. State of the Art
Insulative structures of semiconductor devices have long been formed from silicon-containing materials, such as silicon dioxide and silicon nitride. Silicon dioxide structures are typically fabricated by forming a silicon layer over a semiconductor device structure and oxidizing the silicon layer or by deposition processes that employ materials such as tetraethylorthosilicate (TEOS). Silicon dioxide layers so formed are then patterned by known processes to define insulative structures of the semiconductor device.
Silicon nitride insulative structures may be formed by first forming a layer of silicon on a semiconductor device structure, then nitridating the layer of silicon so as to form a silicon nitride layer. Conventionally, nitridation of silicon has been effected by exposing the silicon to nitrogen-free radicals from sources such as nitric oxide (NO), nitrous oxide (N2O) and ammonia (NH3). Free radicals may be generated from these nitrogen-containing species by use of plasmas. Once a silicon nitride layer has been formed, the silicon nitride layer may be patterned by known processes to form insulative structures of the semiconductor device.
Conventional semiconductor devices typically include conductive lines with thicknesses of at least about 0.25 microns. The insulative structures of the semiconductor devices, which are fabricated by conventional processes, have comparable thicknesses and impart to the semiconductor device the desired dielectric properties.
The trend in the semiconductor industry is toward fabricating semiconductor devices including structures of ever-decreasing size. While the widths of conductive lines of state-of-the-art semiconductor devices are currently in the range of about 0.25 microns down to about 0.18 microns, the widths of conductive lines and, thus, of the insulative structures adjacent thereto, are continuing to decrease. The goal in the industry is to decrease the thicknesses of conductive lines and their adjacent insulative structures down to dimensions that are measurable in terms of a few molecules or even single molecules.
In semiconductor device structures, the silicon-oxide interface contains interface states and surface defects caused by unsatisfied chemical bonds, or xe2x80x9cdangling silicon bonds.xe2x80x9d Unsatisfied bonds in silicon atoms contribute to a charge at the oxide surface and the existence of the interface states cause the threshold voltage to fluctuate.
As the thicknesses of insulative structures of semiconductor devices decrease, dangling silicon bonds at interfaces between insulative structures and adjacent silicon structures, such as source/drain regions (i.e., n-wells and p-wells) and polysilicon conductive structures, become problematic. In conventional semiconductor devices, the dangling silicon bonds are present at concentrations of about 1011 to about 1012 per square centimeter. While these concentrations of dangling silicon bonds do not cause significant problems in conventional semiconductor devices, dangling silicon bonds may cause defects in much thinner insulative structures, which may cause electrical shorting in semiconductor devices including such thin insulative structures and, thus, failure of the semiconductor devices.
A common solution to this problem has been to passivate the interface by exposing the interface to high concentrations of molecular hydrogen (H2), or hydrogen gas, which acts as a source for hydrogen atoms (H). The passivation is effected by rapid thermal processing (xe2x80x9cRTPxe2x80x9d) and furnace tools before encapsulation with a thick nitride film. It is believed that the hydrogen atoms bond to the dangling silicon bonds to passivate the interface. Unfortunately, the use of H2 raises serious safety concerns. The explosive nature of hydrogen gas narrows the window of acceptable process conditions involving its use. Argon or another inert gas can be mixed with H2 to mitigate some of the safety risks, but this dilution reduces the overall rate of reaction, which results in unsatisfactory processing times. Additionally, hydrogen passivation must be done at the end of the processing of a semiconductor device structure. Otherwise, the hydrogen will escape from the interface region.
Moreover, the passivating hydrogen species are more readily driven from the passivated structures when a semiconductor device structure under fabrication is exposed to high process temperatures, such as processes that require temperatures of about 600xc2x0 C. or greater.
Another method that has been proposed for passivating semiconductor device structures during fabrication thereof so as to prevent dangling silicon bonds from causing defects to be formed in insulative structures includes the use of deuterium species derived from molecular deuterium (D2). When deuterium is used to passivate the various structures, including insulative structures, of a semiconductor device structure under fabrication, deuterium species, including deuterium-free radicals, permeate the various structures of the semiconductor device structure. It has been further proposed that by encapsulating deuterium-passivated structures with a suitable material, such as silicon nitride, the concentration of deuterium that permeates, and thus, passivates the semiconductor device structures will not be significantly diminished when the semiconductor device structure under fabrication is exposed to high process temperatures. Further, deuterium has been used to form silicon nitride layers by reaction with silane or dichlorosilane (DCS), ammonia, and molecular deuterium. Nonetheless, the use of molecular deuterium poses many of the same threats as those present during the use of molecular hydrogen.
Thus, it can be appreciated that it would be advantageous to develop a technique for passivating the silicon-silicon dioxide interface by trapping a passivating species at the interface using a method that mitigates the problems present in the prior art.
The present invention includes a method for passivating with hydrogen species derived from ammonia (NH3) semiconductor device structures under fabrication and, particularly, insulative structures of the semiconductor device structures. The method of the invention includes obtaining the hydrogen species from ammonia so as to allow the hydrogen species to permeate the semiconductor device structure under fabrication, as well as the insulative structures thereof, and forming an encapsulant layer over the semiconductor device structure so as to retain the hydrogen species within at least the insulative structures of the semiconductor device structure. The encapsulant may be formed following exposure of the semiconductor device structure to the hydrogen species or concurrently with exposure of the semiconductor device structure to the hydrogen species.
Hydrogen species, including hydrogen-free radicals, may be derived from ammonia by known processes. High temperature processes, for example, known rapid thermal processing (RTP) and other batch system techniques, may be employed. As hydrogen species from the ammonia permeate the semiconductor device structure, the concentration of dangling silicon bonds present at interfaces between silicon and silicon oxide structures is decreased from about 1011 to about 1012 dangling silicon bonds per square centimeter to about 109 to about 1010 dangling silicon bonds per square centimeter, or to about 1% of the original concentration of dangling silicon bonds present at these interfaces. This represents a decrease in concentration of dangling silicon bonds of about two orders of magnitude.
As the passivating hydrogen species that permeate the semiconductor device structure, including the interfaces between the insulative and semiconductive structures thereof, tends to escape the semiconductor device structure upon exposure thereof to temperatures of greater than about 400xc2x0 C. to greater than about 600xc2x0 C., the method of the present invention preferably includes forming an encapsulant layer over hydrogen-passivated portions of the semiconductor device structure. The encapsulant layer may be formed in situ with the hydrogen passivation or in separate process equipment. Known techniques may be used to form the encapsulant layer. By way of example and not to limit the scope of the present invention, the encapsulant layer may comprise silicon nitride and may be formed by known techniques, such as by chemical vapor deposition (CVD) or by nitridation of a silicon layer. As a first example of the method of the present invention, the encapsulant layer may be formed directly over an insulative structure that forms an interface with a semiconductive structure for which hydrogen passivation is desired. In a variation of the method of the present invention, the encapsulant layer may be formed over additional structures that are formed subsequent to the fabrication of the insulative structure that forms an interface with a semiconductive structure, but before high temperature processes that would drive the passivating hydrogen from the semiconductor device structure are conducted. In another variation of the method, the encapsulant layer may be formed substantially simultaneously with the disassociation of ammonia to provide hydrogen species that will permeate and passivate the semiconductor device structure. In any event, the encapsulant layer preferably substantially completely covers a large-scale substrate, such as a semiconductor wafer, upon which a collection of semiconductor device structures are being fabricated.
Once a semiconductor device structure has been passivated and encapsulated in accordance with teachings of the present invention, high temperature processes may be effected. The hydrogen passivation and the accompanying encapsulation reduces or eliminates the occurrence of defects in thin insulative structures that could be attributed to exposure of dangling silicon bonds at interfaces between the insulative structures and adjacent semiconductive or conductive structures to temperatures of about 400xc2x0 C. or greater. While some of the hydrogen species may escape through the backside of the semiconductor device structure or even through the encapsulant layer, the encapsulant layer is said to substantially contain the hydrogen species, or to substantially prevent the hydrogen species from escaping the semiconductor device structure.
Following the completion of high temperature fabrication processes on the semiconductor device structure and, thus, once the threat of defects being introduced into the insulative structures of the semiconductor device has passed, portions of the encapsulant layer may be removed so as to facilitate further fabrication of the semiconductor device structure.
The method of the present invention is useful in fabricating a variety of types of semiconductor device structures that have conductive lines and insulative structures of relatively small dimensions. By way of example and not to limit the scope of the present invention, the hydrogen-passivation method of the present invention may be employed in fabricating transistors, capacitors, polysilicon resistors, and thin-film transistors (TFTs), or polysilicon transistors.
In another aspect, the present invention includes methods for preventing unwanted voltage shifts, or changes across dielectric structures, such as the gate oxides of transistor gates of the dielectric layers of capacitors. As is known in the art, the presence of certain threshold voltages, or potential differences, across dielectric layers of such structures is desired. Unwanted changes in the voltage across a dielectric layer may occur if current leaks across the dielectric layer. For example, as the ratio between the amount of current that is required in changing the state of a transistor gate of a thin film from xe2x80x9coffxe2x80x9d to xe2x80x9conxe2x80x9d is very small relative to the current ratio needed to shift a traditional semiconductor transistor gate from an xe2x80x9coffxe2x80x9d state to an xe2x80x9conxe2x80x9d state, the presence of dangling silicon bonds above certain concentrations at the interfaces between insulative structures and semiconductive structures of thin-film transistors may cause defects that blur the distinction between the xe2x80x9coffxe2x80x9d state and the xe2x80x9conxe2x80x9d state of the thin-film transistor. By employing the method of the present invention to passivate at least the interfaces between insulative structures and semiconductive structures of thin-film transistors, the concentration of dangling silicon bonds present at these interfaces is effectively reduced, as is the likelihood that the dangling silicon bonds could cause defects.
The present invention also includes semiconductor device structures with semiconductor-insulator interfaces that are exposed to disassociated ammonia species that are substantially contained by an encapsulant layer. Preferably, the concentration of dangling silicon bonds present at the encapsulated semiconductor-insulator interfaces of such a semiconductor device structure is about 1010 to about 109 or less dangling silicon bonds per square centimeter.
In addition, the invention includes passivating structures that include disassociated ammonia species in the presence of an interface between a structure comprising semiconductive material silicon and an adjacent insulating structure, as well as an encapsulant layer substantially containing the disassociated ammonia species at least at the interface.
Other features and advantages of the present invention will become apparent to those of skill in the art through a consideration of the ensuing description, the accompanying drawings, and the appended claims.